Place saute pan on high heat for 4 minutes. Sear short ribs on all sides, 4-5 minutes total.
Place short ribs, soy sauce, 1/4 cup of the vinegar, garlic, brown sugar, and gochujang into slow cooker. Cook on high for 4-6 hours.
As meat cooks, thinly slice onion. Place sliced onion into small pot with salt, white sugar, remaining 3/4 cup of vinegar, and 1/2 cup of water. Bring to a simmer, then immediately cool.
Once meat is cooked, remove from slow cooker and whisk in cornstarch (premixed with a splash of water). Shred short ribs and serve on tortillas, garnished with the picked onion, green onion, mayo, and a splash of the braising liquid.